Protective device



Sept. 7, 1937.

R. S. HOLM ES PROTECTIVE DEVICE Filed Feb. 28, 1934 @L i j /lV/'Jl/T'AVAVAVA i7 llmlllml Patented Sept. 7, 1937 UNITED STATES smear ortiesPROTECTIVE DEVICE Delaware Y Application February 28, 1934, Serial No.713,280

8 Claims.

My invention relates to protective devices and particularly to devicesfor preventing destructive currents from flowing in thermionic tubes andthe like.

In cascade amplifiers of the so-called direct current type, the grid ofone thermionic tube is conductively connected to a resistor included inthe output circuit of a preceding tube, the potential developed acrossthe resistor, when the amplifier is in operation, being depended upon tosupply a negative bias to the said grid. If the tubes are of theequipotential cathode type, which require a relatively long time to warmup, the bias on the grid is positive during the warming up period anddestructive current is prone to flow therein. The flow of current duringthe warming up period is particularly harmful to cathode ray tubes, ofthe type now being utilized in television receivers, since it may spoilthe emissivity of the cathode.

It is, accordingly, the principal object of my invention to provide aprotective device, for use in connection with direct current amplifiers,that shall function to protect the thermionic tubes against injury,during the warm up period and also when one of the tubes is removed fromits socket.

At the present time, one of the important fields of usefulness for myinvention is in connection with amplifiers of the type wherein all anodeand grid-biasing potentials for a plurality of thermionic devices aresupplied from a single bleeder resistor connected between the terminalsof a source of unidirectional potential. In an amplifier, according tomy invention, I interrupt the continuity of the bleeder resistor byinterposing between the terminals thereof the space current path in athermionic device the cathode of which is supplied with heatingpotential from the 4 same source that supplies the cathodes of the otherthermionic devices in the amplifier. The interposed thermionic devicewill be referred to, hereinafter, as the delay tube.

Preferably, the cathode of the delay tube heats no faster than thecathodes of the amplifier tubes although, as will be pointed out later,it is not detrimental to utilize a cathode that heats at the sameV rate.By reason of the interposition of the delay tube, current is preventedfrom flowing in Va portion of the bleeder resistor until the amplifiertubes have been supplied with sufcient negative grid-biasing potentialto prevent the flow of destructive currents therein.

55 The novel features that I consider characteristic of my invention areset forth with particu- CII (Cl. Z-27) larity in the appended claims.The invention tself, however, both as to its organization and its methodof operation, together with additional objects and advantages thereof,will best be understood from the following description of a spec'icembodiment, when read in connection with the accompanying drawing, inwhich:

Fig. 1 is a diagrammatic view of a direct coupled amplifier including anembodiment of my invention,

Fig. 2 is a diagrammatic View of a direct coupled amplifier including amodified embodiment of my invention, and

Fig. 3 is a diagrammatic view exemplifying the application of myinvention to a television receiver including a cathode ray tube.

Referring now to Fig. 1 of the drawing, the usual direct coupledamplifier includes a plurality of thermionic devices, exemplified in thedrawing by the tubes I and 3, which are supplied with 20 anode andgrid-biasing potentials from a common bleeder resistor 5. From aninspection of the drawing, it will be noted that the tube 3 receives itsgrid-biasing potential from a resistor 'l included in the output circuitof the tube i. In view of the fact that such bias, if not compensated,might well be much too negative, an adjustable connection S is usuallyinterposed between the cathode of the biased tube and a point on thebleeder resistor more negative than the point to which the bias-supplyresistor 'l is connected.

Assume, for the moment, that the continuity of the bleeder resistor 5 isnot interrupted when the system is first energized, as by closing aswitch H in a potential supply lead connected to the positive terminalof the bleeder resistor. During the Warming up period, the first tube Iin the series draws but little anode current and the bias on the grid ofthe second tube is positive -with respect to its cathode. Such being thecase, destructive current may possibly flow in the tube 3 unlessautomatically functioning means are provided to prevent.

According to my` invention, therefore, I provide means for preventingexcessive current from flowing in the biased tube during the warmupperiod such means being constituted by a delay tube I3, the spacecurrent path in which is interposed in series with the bleeder resistor.Specifically, I sever the bleeder resistor between the connectionthereto of the cathodes of the several tubes and connect the delay tubebetween the severed portions. For the delay tube I prefer to utilize adiode although a triode may be utilized, as illustrated, in which eventthe grid thereof may be supplied with bias by a connection l5 to thebleeder resistor. If the potential supply to the bleeder resistor isquite variable, the triode offers some advantages in that it tends tohold. the current in the second tube constant.

During the warming up period, with the delay tube interposed asillustrated in the drawing, the grid and cathode of the tube 3 are atsubstantially the same potential and minimum current flows therein untilthe cathode of the delay tube reaches full emission temperature. By thattime, the first tube l has also reached emission temperature and, byreason of normal anode current fiowing in the output resistor, theprotected tube 3 has been supplied with normal negative bias.

It also lies within the scope ofV my invention to dispense with aseparate delay tube and to utilize, instead, a diode plate associatedwith the cathode of the first tube in the series. Such modification isillustrated'in Fig. 2 of the drawing, in which it will be noted that adiode plate il has been disposed within the envelope of the first tubel, adjacent the cathode thereof. Since the operation of the modifiedprotective system is identically the same as that exemplified by Fig. 1,no necessity is seen for going into further detail concerning it. Thesystem shown in Fig. 2 has the still further advantage of affordingprotection to the second tube 3 in the event that it is necessary toremove the first tube i from its socket in case of failure. The reasonfor this is obvious.

As hereinbefore pointed out, it is highly essential that cathode raytubes, in television receiving circuits and the like, be adequatelyprotected against excessive beam-currents. The manner in which this maybe accomplished according to my invention is illustrated by Fig. 3 ofthe drawing, wherein is shown a thermionic amplifier tube i9 of thetriode-diode type, corresponding to the tube l in Fig. 2, having aresistive output circuit 2l directly connected'to the ray-controlelement, or grid 23, in a cathode ray tube 25. As in the other figures,both tubes are shown as being supplied with anode potential from acommon potential source Z'i. Separate sources, of course, may be used.As described in connection with the previous figures, grid biasingpotential for the cathode ray tube is supplied from the output circuitof the preceding amplifier tube I9. As a g-eneral rule, such potentialis much too negative for proper operation of the cathode ray tube if thecathode is directly connected to the positive terminal of the amplifieranode potential source. For that reason I prefer to connect the cathodeof the tube to the movable contact element 29 of a potentiometer, theresistor element 3l of which is included in a circuit, shunting thatportion of the potential supply source which feeds the anode of theamplifier tube. The shunting circuit must not pass current during thewarm up period when the amplifier tube i9 is not taking plate currentsince this would cause the cathode of the cathode ray tube to becomenegative with respect to its grid and would permit destructive currentto fiow. Accordingly, I interpose a space current path in the shuntingcircuit, the impedance of which changes in accordance with the impedanceof the space current path in the amplifier tube. Specifically, suchspace current path is constituted by the cathode in the amplifier tubeand by an eXtra diode plate 33 therein. Obviously, no current can flowin the shunting circuit until the amplifier tube is passing current byreason of the heating of its cathode and the protective object of myinvention is thus accomplished.

From a consideration of the foregoing it will be apparent that I haveprovided a novel and useful protective system particularly adapted toamplifiers of the direct coupled type. The modifications chosen forpurposes of explanation, however, are merely indicative of the many usesto which the invention may be put, and my invention is not to be limitedexcept as necessitated by the prior art and by the spirit of theappended claims.

I claim as my invention:

1. In combination a multitube amplifier, a thermionic device having acathode and an output electrode, a thermionic device having a cathodeand an input electrode, a direct conductive connection between the saidelectrodes, means including a source of power for generating a directcurrent potential VVand for supplying it to said electrodes, and aconnection between the cathodes of the first mentioned devices, the saidconnection including the space-current path in a thermionic device.

2. The invention set forth in claim 1 additionally characterized in thatthe space current path is defined by the cathode in the thermionicdevice having the output electrode, and a diode plate in the saiddevice.

3. The invention set forth in claim 1 additionally characterized in thatthe last named connection includes a variable bias source for saidsecond thermionic device.

4. In combination, a thermionic device of the hot cathode type having agrid-electrode, a thermionic amplifier device, also of the hot cathodetype, having an anode, a direct connection between the anode of the lastmentioned device and the grid of the first mentioned device, a resistor,

means for applying potential to said anode through said resistor wherebya plate current flow through said last mentioned device produces apotential drop across said resistor, means for applying said potentialdrop to said gridelectrode as a negative bias, and protective means bothof said devices have substantially the same thermal lag.

7. In combination, a thermionic device of the hot cathode type having agrid electrode, a thermionic amplifier device, also of the hot cathodetype, having an anode, a direct connection between the anode of the lastmentioned device and the grid of the first mentioned device, protectivemeans for preventing the grid of the first mentioned device frombecoming positive with respect to its cathode when energizing potentialsare first applied to both devices, said protective means including aspace current path in said amplier device between the cathode thereofand a diode-plate therein.

8. In combination, a cathode-ray tube having a control grid and acathode which is electron emissive when heated, an electric dischargetube having an anode and a cathode which is electron emissive whenheated, a direct conductive connection between said anode and saidcontrol grid, a resistor, means for applying potential to said anodethrough said resistor whereby a plate current iiow through said electricdischarge tube produces a potential drop across said resistor which isapplied to said control grid as a nega- Y tive bias, a device throughwhich current ows readily only when heat is applied thereto, said devicehaving at least as much thermal time lag as the cathode of saiddischarge device, and a connection between said cathodes which includessaid last device.

RALPH S. HOLMES.

